The drives that the backup is using are virtual tape drives from our VTL, which are empty. I am also able to mount and dismount the tapes without any issues.
Another strange issue we have is that the drives often go offline, without reason.

There seems to be some communication issues here too - mention of unexpected message from NDMP and a lot of name resolution going on - which seems to resolve host hames as IP addresses rather than names.

Worth making sure you Master and Media Servers can reolve each other and the filer by short and FQDN name - use hosts files if neccesary.

Odd to have robot / tape errors on a VTL so most likely a communications / configuration issue.

Run through you config again and to be sure of everything and ask the NDMP admin for the messages log from the filer to see if that is showing any errors - also make sure that the fibre switches involved are not showing any issues (some switches will cause a port reset if any error is detected which can disconect drives)

Finally take a look at the event log on the VTL to see if that is reporting anything

This may all be a communications or connection issue, but if it centres around the NDMP then that is where you need to look

Hope this helps

Authorised Symantec Consultant

Don't forget to "Mark as Solution" if someones advice has solved your issue - and please bring back the Thumbs Up!!.

A 98 has a lot of causes, but NBU itself is usually not one of them.
I wouldn't go insane looking at BPTM logs, or Daemon/LTID logs.. I'd rather check configurations, and HBA/FC Cables/SFPs.

The things I'd tell you to check when you get a 98 are:

1) Make sure the TLD robot inventory is up to date.
2) Make sure the drive path is correct and visible. Check "storage show tape" output on the filer, and match the serial numbers shown with the paths configured in NetBackup. ( or run "tpautoconf -probe <filer>" from your NDMP Media Server, then "tpconfig -dl" to match the devices )

3) Are you 100% possitive that the correct DRIVE INDEX is assigned on NetBackup?
NetApp VTLs start on Drive Index 0, while NetBackup starts on Drive Index 1...